Graphic arts applications frequently require the accurate reproduction of a high resolution color image (i.e. an artwork), such as a photograph, a color drawing, a color layout and the like. A typical application might involve printing a high resolution color image on a page of a periodical, such as a magazine, or a corporate annual report.
Color images are oftentimes generated either photographically, on suitable film, or electronically, on video tape or other suitable electronic media. When generated, these images share a basic characteristic: they are recorded on a continuous tone (hereinafter referred to as "contone") basis. As such, the color existing at any point in the image is recorded by an amplitude value, either exposure in the case of film or a voltage level in the case of electronic media.
Color reproduction equipment takes advantage of the principle that any color can be separated into a specific linear combination of four primary subtractive colors (cyan, yellow, magenta and black--CYMB) in which the amount of each primary color is set to a predetermined amount. In the case of printed reproductions of an image, use of primary color printing obviates the need to use a differently colored ink for each different color in the image. As such, each image is converted into a succession of three or four color separations, in which each separation is essentially a negative (or positive) transparency with an altered tone reproducing characteristic that carries the color information for only one of the primary colors. A separation is frequently made by photographing or electronically scanning an artwork through an appropriately colored filter. If, illustratively, a digital scanner is used, then each resulting contone value is frequently an eight binary bit number that represents the relative intensity of a corresponding primary color between a range of "0" (no intensity) to "255" (full intensity). A set or file of corresponding contone values would then exist for each separation.
Unfortunately, modern printing presses do not possess the capability of applying differential amounts of ink to any location in an image. Rather, these presses are only designed to either apply or not apply a single amount of ink to any given location. Therefore, a printing press is unable to directly print a contone separation. To successfully circumvent this problem, halftone separations are used instead. An image formed from halftone separations encodes the color information inherent in a color image from amplitude modulated form into a spatial (area) modulated form, in terms of dot size, which is subsequently converted by the human eye into a desired color. Specifically, it has been known in the art for quite some time that, for black and white images, a number of small black dots of a corresponding size, when printed over an area and later viewed at a distance, will be spatially integrated by human eye into an intermediate shade of grey. The size of the dot can be varied from 100%, i.e. a full dot, through 50%, a half dot, to 0% (at which no dot is printed) to yield the color black, gray or white. Hence, by smoothly changing dot sizes (areas), smooth corresponding tonal variations will be generated in the reproduced image. Given this, the art has taught for some time that a full color image can be formed by properly overlaying single color halftone reproductions for all of the primary subtractive colors, where each reproduction is formed from a halftone dot separation that contains dots of appropriate sizes and in one of these primary colors. Clearly, as size of the dots decreases, an increasing amount of detail can be encoded in a dot pattern and hence in the reproduced image. For that reason, in graphic arts applications, a halftone separation utilizes very small dots to yield a dot pitch (resolution) ranging from 85 to as much as 200 dots/inch (approximately 33 to 79 dots/centimeter).
With this in mind, one would at first blush think that printing a color image for graphic arts use should be a fairly simple process. Specifically, a color image could first be converted into corresponding continuous tone separations. Each of these contone separations could then be converted into a corresponding half tone separation. A printing plate could then be manufactured from each halftone separation and subsequently be mounted to a printing press. Thereafter, paper or other similar media could be run through the press in such a fashion so as to produce properly registered superimposed halftone images for all the subtractive primary colors thereby generating a full color reproduction of the original image.
Unfortunately, in practice, accurately printing a color image is oftentimes a very tedious, problematic and time consuming manual process that requires a substantial level of skill. First, the process of converting a contone separation into a halftone separation, this process commonly being referred to as "screening", is a time consuming manual process in and of itself. Second, various phenomena, each of which disadvantageously degrades an image, often occur in a reproduced halftoned color image. Moreover, the complete extent to which each of these phenomena is present in the reproduced image is often known only at a rather late point in the printing process thereby necessitating the use of tedious and time consuming iterative trial and error experimentation to adequately eliminate these phenomena.
Screening is traditionally accomplished photographically. Where each color separation in the form of negative type transparency has been made (i.e. "indirect color separation photography"), a separate screening step is performed to yield a halftone positive. Specifically, a contact screen, now typically a flexible transparency (such as illustratively a positive type KODAK Magenta Contact screen or a KODAK MARK MLR type contact screen) that contains a precise two dimensional grid-like pattern of vignetted dots (having a sinusoidal density pattern) at a resolution of 85 to 200 lines/inch (approximately 33 to 79 lines/centimeter), is placed in direct intimate contact over the emulsion side of a piece of high contrast orthochromatic film, such as KODALITH Ortho film 2556, type 3 (KODAK, KODALITH and MARK MLR are trademarks of Eastman Kodak Company), in a vacuum holder located in the back of a suitable vertical process camera. These films are commonly referred to as "lith" films. As such, incoming light will pass into the camera and then through the contact screen to selectively expose certain areas on the film. A color separation is positioned on the copyboard of the process camera such that the center of the separation is centered over the lense of the camera. Thereafter, light is projected onto the separation to uniformly illuminate the film plane in the process camera. The camera lense is then opened, and a suitable time exposure is made. Each area on the lith film where light was blocked by a dot in the screen will, with a positive type screen, remain white when developed to yield a positive halftone image. All other areas in the lith film that have been exposed to light passing through this screen will become black when developed. In lieu of using a process camera, a vacuum printing frame can be used where the separation transparency is mounted against one side of the screen with the lith film abuttingly mounted against the other side of the screen and light being shined directly through the separation, screen and onto the emulsion side of the lith film. In any event, the amount (density) of light passing through the screen and reflected from or passing through a contone separation at any given location will determine the size of the resulting dot (circular or square with a conventional screen, or elliptical with an elliptical screen) that will occur at a corresponding location on the developed lith film. The lightest areas on the separation will reflect or transmit therethrough the most light and will produce the largest dots. The darkest areas on the separation will reflect or transmit therethrough the least light and hence will produce the smallest dots. Once the lith film has been appropriately exposed, it is then chemically developed to yield a halftone separation. This entire photographic process is then repeated for each of the remaining color separations to generate the remaining halftone separations. Alternatively, in direct color separation photography, separate screened negative type color separations are directly generated from an original color image rather than from a color separation. Here, an appropriate color filter is typically placed onto the lense of the process camera to separate out a primary color from the original image. A negative type screen, such as a KODAK Gray Contact Screen (Negative type) or a KODAK MARK GSR screen for generating reflection copy with a process camera or a KODAK MARK GMR screen for use with a color transparency of the image mounted in a vacuum mounting frame, can be used to directly generate the negative halftone separation from the image (MARK GSR and MARK GMR are trademarks of the Eastman Kodak Company). The remainder of the screening process is substantially identical to that discussed above.
Unfortunately, photographic screening processes possess several major drawbacks. First, because of the large number of manual steps involved, photographic screening is time consuming. Second, a camera operator that performs this process must possess a very high level of skill in order to obtain accurate results. In particular, the operator must accurately regulate the exposure light to assure that lith film is evenly illuminated. Additional exposures, such as shadow and highlight exposures, of a lith film may also be necessary to achieve proper tonal rendition in a halftone separation. The operator also needs to recognize and compensate for the fact that subtle differences in the reproduced image may arise if he changes from an old screen to a new screen in which the latter has smaller tolerances and hence a more sharply defined dot pattern than the former screen or if he changes from one type of lith film to another. Furthermore, the operator must reduce all stray non-image forming light (commonly referred to as flare) that reaches the lith film to a minimal amount. Moreover, the operator must use proper techniques in developing the exposed lith film in order to assure that acceptable tonal rendition occurs in the reproduced color image.
In addition, the dot patterns existing in each of the superimposed halftone reproductions in a full color image frequently interact with each other to produce a low frequency spatial beat (interference) pattern that appears as a repeating rosette pattern in the image and is commonly referred to as a Moire pattern (hereinafter referred to as Moire). If the beat pattern is sufficiently low, Moire is very visible, quite unsightly and highly objectionable to a viewer. To reduce visible Moire, each halftone separation is produced at a different screen angle. The screen angle is defined as the angle between the rows of dots on the screen and the vertical (or horizontal) axis of the lith film or the scanned image. For four color (CYMB) printing, screen angles of 45, 75, 90 and 105 degrees for black, magenta, yellow and cyan screens, respectively, are commonly used. Either the individual screens can be suitably rotated or pre-angled screens, such as KODAK Pre-Angled Gray or Magenta contact screens, can be used. In either case, use of differently angled screens shifts the beat patterns to a relatively high frequency where the Moire is far less noticeable, if at all, to a viewer. While such preset screen angles are often used, the amount of Moire that actually exists in any printed full color image is not known until that image is actually printed.
In addition to screening induced Moire, a full color image can contain undesirable artifacts, such as spots, streaks or the like, that also need to be removed. Often this entails that the coloration of a region of one or more of the separations or of the underlying contone image itself needs to be changed, by one or more techniques, such as tinting, opaquing or "air brushing", to eliminate the artifact. Unfortunately, it is frequently not known a priori, until a point is reached much later in the color printing process, i.e. when a proof and oftentimes a press sheet, as described below, is analyzed, whether use of any of these techniques will effectively remove the artifact and, if such a technique is selected for use, the full extent to which it should be used in order to satisfactorily remove the artifact.
During printing, dot gain also presents a problem. Printing paper absorbs ink. As such, whenever a dot is printed on a page, the ink used to print that dot diffuses into a printed page and as a result creates a slightly larger dot than that intended on the surface of the page. This affect is exacerbated inasmuch as ink is forced against the page by impression pressure exerted onto the page by the printing press. In any event, the visual enlargement in area covered by a dot on a printed page over the area contained in a corresponding halftone separation is referred to as dot gain. Frequently, dot gain is not uniform over all dot sizes and is greatest for dot sizes lying between 30-70% (middletones). Dot gain variations frequently occur between different types of paper (least for a good coated stock, increasing for an uncoated stock and greatest for a soft stock such as newsprint), different presses, and due to normal press variations between different presses of the same type and/or the same press being operated on different days. Therefore, to generate an accurate press sheet, smaller dots, i.e. dots reduced in size by the applicable dot gain, than those that would otherwise be contained in the halftone separations must be generated during screening. Unfortunately, due to normal press variations, the exact amount of dot gain that is expected is generally not known until an actual press sheet is run.
To verify the accuracy of the color printing process and to enable appropriate adjustments to be made at various stages in the printing process in order to correct image defects and improve reproduction accuracy, a test image, frequently referred to as a "proof" is generated from the halftone separations once they are made. After a proof is generated, it is presented as being representative of the reproduced image which will be produced by a printing press in order to determine the accuracy of the printed image. Oftentimes, the proof contains unexpected and unsightly Moire patterns that arose from the interaction of Moire in the image itself with that introduced by use of the angled screens. Frequently, these Moire patterns can be rendered invisible by further rotation of one or more of the screens to a different screen angle. Unfortunately, the exact change in the screen angle is frequently very hard to discern from the resulting Moire pattern itself and instead must be determined through trial and error experimentation. Unexpected artifacts can also exist in the proof thereby necessitating that various changes must be made to one or more of the separations. As such, this requires that a one or more new halftone separations must be generated or at least changed, a new proof must be produced and then analyzed, with this "proofing" process being iteratively repeated until the objectionable Moire and all objectionable artifacts are eliminated from the proof. Now, once an acceptable proof is made thereby indicating that a printed image based on the separations will likely present a desired depiction of the original artwork, a separate printing plate, such as an aluminum sheet with an appropriate organic film coating, is then made for each halftone separation. At this point, a full color test print, commonly referred to as a "press sheet", is produced from these plates onto a sheet of actual paper stock that is to be used to carry the reproduced image, with this operation frequently being referred to as a "press run". The press sheet is then examined to discern all imperfections that exist in the image reproduced therein. Owing to unexpected dot gain, existence of any artifacts in the press sheet and tonal variations occurring in the press run between the press sheet and the original artwork, further adjustments in the coloration or screen angle of the separations may need to be made with the entire process, i.e. both the proofing and the press run processes, being repeated until an acceptable press sheet is produced. Specifically, if a portion of the artifact remains in the press sheet after one or more of the separations has been changed as set forth above or if an artifact is introduced through interaction of the superimposed halftoned reproductions that form the press sheet, then one or more of the separations may again need to be changed and the entire process iteratively repeated until an acceptable press sheet is produced. Frequently, this change in one or more of the separations is made along with a change in screen angle to eliminate any unsightly Moire. With experience gained over several years, a skilled color technician can reduce the number of times that this entire process needs to be repeated in order to produce a set of color halftone separations that yields an acceptable press sheet.
As one can now readily appreciate, the iterative manual process of producing an acceptable set of halftone separations, due to the inherent variability of the process, can be very tedious and inordinately time consuming. Unfortunately, in the graphic arts industry, publication deadlines are often extremely tight and afford very little, if any, leeway. Consequently, the available time in a graphic arts production environment allotted to a color technician to generate a set of halftone separations to meet a particular publication deadline, for example, is often insufficient to allow the technician adequate time, due to the trial and error nature of iterative process, to generate that set of separations which produces a very high quality halftoned color image. As such, the technician is often constrained by time pressures to produce a set of separations that produces a visually acceptable, though not necessarily a very high quality, image.
In addition, the manual process can be disadvantageously quite expensive. Inasmuch as the manual process, even for a skilled color technician, involves a certain amount of trial and error experimentation, a number of separate proofs is often made with changed or new separations being generated as a result. Each new separation requires another piece of lith film. Film and associated developing chemicals are expensive. In addition, if an unacceptable press sheet is produced, then additional separations may need to be made along with new printing plates, which further increases the expense of the process.
In an effort to reduce the time required and expense associated with manual photographic based color reproduction processes, the art has turned away from use of these manual processes in high volume graphic art applications to the use of electronic image processing systems. These systems convert contone images or separations into electronic (often digital) form, electronically change screen angles and compensate for expected dot gain, electronically produce appropriate halftone separations and in some instances halftoned color images directly onto a sheet of paper thereby eliminating the need to photographically generate both separation transparencies and a proof. Through use of electronic image processing, these systems aim to produce high quality halftoned color images at a substantially increased throughput than that possible with traditional manual processes. However, for various reasons as discussed below, these electronic systems when used in graphic arts production environments often fall short of tis goal.
An essential component of an electronic image processing system is an electronic screener that converts contone images or separations into corresponding halftone separations for subsequent use in directly driving an electronic dot printer. Specifically, these systems frequently utilize a high resolution marking engine, such as a laser printer, which prints writing spots of a single color at a resolution of illustratively at least 3000 spots/inch. Through such a marking engine, each halftone dot is formed as a group of writing spots. Inasmuch as the color of a toner used in the laser printer specifies the color of each spot, a separate toning pass is made through the printer for each of the colors cyan, yellow, magenta and black using a correspondingly colored toner to generate each separate halftoned image that is used in collectively forming a full color image.
Various techniques for electronic screeners exist in the art but each of these techniques suffers from one or more drawbacks. In particular, United States patent 4,727,430 (issued to M. Miwa on Feb. 23, 1988) discloses one such technique for generating halftone images from an original image. Here, the screen that is used for any area of the original image is selected based upon both the image content, specifically density and hue information, occurring at that area and the coordinate location of that area in the original image. Unfortunately, by first establishing separate comparison ranges of density and hue and coordinate location information for the entire scanned image and then comparing each area in the image to the values for each of these ranges in order to select the proper screen for that area, this technique is likely to be very slow which, in turn, will disadvantageously limit the throughput of an electronic image processing system that uses this technique.
Another electronic screening technique is disclosed in U.S. Pat. No. 4,447,831 (issued to D. E. Adsett et al on May 8, 1984 and hereinafter referred to as the '831 Adsett et al patent). This technique, executed in software, relies on first dividing an image into pre-defined areas of 128 adjacent pixels, then generating a weighting function for each 32 pixel sub-area in every area in the image, algebraically combining the weighting functions for each sub-area of each corresponding area to determine the angular modulation present in that area, encoding each area with a value indicative of angular modulation associated therewith, and finally, in response to the encoded value, selecting an appropriate halftone dot pattern to be printed for that area. First, this technique appears to be limited to printing relatively large halftone dots rather than relatively small dots as required in graphic arts applications. Second, the relatively large number of steps that must be performed to select a halftone dot pattern for every area in an image will likely cause this technique to disadvantageously require an excessively large amount of time to process an entire image. In particular, a screened halftone bit-mapped image destined for reproduction by a high resolution marking engine, such as a laser printer, may easily contain in excess of 100 Mbytes of data/separation with a resulting color image containing upwards of 16 Mbits of color information per square inch (or approximately 2.5 Mbits/square centimeter) of image area. Consequently, any software based screening technique, such as that disclosed in the '831 Adsett et al patent, will likely require an inordinately large and hence unacceptable amount of processing time to manipulate the sheer volume of data needed to generate a high resolution screened image. Thus, use of any software based screening technique will disadvantageously highly limit the throughput of an electronic image processing system that uses such a technique and possibly severely degrade its utility for an intended use in a graphic arts production environment.
A different screening technique is disclosed in U.S. Pat. No. 4,419,690 (issued to P. Hammes on Dec. 6, 1983). Here, a halftone separation is generated by helically scanning a color original using an opto-electronic multi-beam scanning head and helically moving a laser completely throughout an output surface, e.g. a separation film, in synchronism with the movement of the scanner. Digital position values are generated to define the current location of the scanning head as it moves along each scanning line in the color original. These values, having X and Y components, are incremented by appropriate horizontal (.DELTA.X) and vertical (.DELTA.Y) increments as defined by the screen angle, output dot pitch and output line spacing. The X and Y components for each successively occurring position are computed by repeated addition of the .DELTA.X and .DELTA.Y increments to the previous positions. Once computed, the position values are used as an address to a memory that stores a pattern of a corresponding overlaid halftone dot situated within an "imaginary" screen and inclined at a given screen angle, .beta., to the horizontal axis of the separation. If the output of the memory reveals that the current position of the scanning head, as it moves along a scanning line, lies within the overlaid halftone dot area, then the position signal is compared against the tonal value of the original image at the current location of the scanning head to determine whether the laser will be pulsed on at that location. If, alternately, the current position of the scanning head lies outside of an overlaid dot area, then the laser remains off at this position. The increments are less than the size of an overlaid halftone dot such that the laser can produce several dots within the area of an overlaid halftone dot. This patent teaches that to reduce visible Moire in the reproduced image: (a) the values of the .DELTA.X and .DELTA.Y increments should be periodically varied as dot borders are crossed, i.e. presumably as the scanning head traverses areas on the original that are associated with adjacent overlaid dots, in order to generate an intermediate screening angle, and (b) the width of the memory that stores an overlaid dot pattern should be chosen such that the width of the overlaid dot is a multiple of the number of scanning beams. Unfortunately, artifacts may be disadvantageously introduced into the separations by the screening process if the .DELTA.X and .DELTA.Y increments are periodically varied during a screening operation. In addition, this technique is disadvantageously somewhat inflexible due to the need to size the memory (overlaid dot shape) appropriately to eliminate Moire in different screening applications.
An additional technique, which relies on representing each repeating halftone cell within a screen by a rectangular matrix and then appropriately combining each pixel within the matrix with pictorial information for spatially corresponding pixels located within an image, is described in U.S. Pat. No. 4,185,304 (issued on Jan. 22, 1980 to T. M. Holladay). Unfortunately, this technique can only be used with certain screen angles thereby failing to suppress certain Moire patterns caused by the mere superposition of all the halftone separations.
Hence, electronic screeners known in the art are often disadvantageously characterized by one or more deficiencies: rather slow operation--particularly those that rely on use of a software based screening technique, operation over limited screen angles, introduction of unwanted artifacts or inflexible operation. Unfortunately, any of these deficiencies tends to limit the throughput of an electronic image processing system that might utilize such a screener. Therefore, an need exists in the art for a hardware based electronic screener that does not substantially possess any of these deficiencies. The use of such a screener will aid in meeting an overall need in the art of providing an electronic image processing system, particularly one destined for use in graphic arts production environments, that has a substantially increased throughput of high quality halftoned color images over that attainable with traditional manual color reproduction processes.